The present invention relates to an emission computed tomography (ECT) apparatus adapted to obtain a tomogram image of radioisotope (RI) distribution by detecting RI distributions in one plane of a subject on the dosage of Ri, from various angles, and reconstructing RI distribution data.
In an ECT apparatus, while rotating a detector constituting a scintillation camera stepwise or continuously around a subject to be inspected, RI distribution data in the subject are collected from various directions in one plane. The detector used in the ECT apparatus basically comprises a parallel-hole collimator, scintillation crystal and photomultipliers, and is used as a two-dimensional position detector. A tomogram image of the RI distribution in the subject is obtained by reconstructing projection data (slice data obtained from a portion of a certain width, or slice portion in a two-dimensional position detector) concerning the RI distribution collected from various angles in one plane of the subject in accordance with convolution and back projection methods, like X-ray CT (computed tomography) apparatus. The reconstructed image is visually displayed by a display device. When there is unequal detection characteristics in a slice of the detector, resulting from the machining accuracy in the collimator and the difference of sensitivities of the photomultipliers, it is inevitable to generate a ringshaped false image (it is called "an artifact" in this technical field) on a tomogram image displayed on the display device. The ring shape of the false image is derived from the data reconstruction in accordance with the back projection processing of slice data collected from various angles of the detector with respect to the subject. The radius of the ring-shaped false image corresponds to a distance between the center of the slice portion in the detector and a portion representing the sensitivity difference. When there are a plurality of portions representing the nonuniformity in detection characteristics in a slice portion of the detector concentric false images are generated.
In order to eliminate false images resulting from the unequal detection characteristics in the detector, a low-pass filter may be used for signal processing. However, this lowers the resolution of a tomogram image.